The present invention relates generally to systems and methods for transmitting data. More specifically, the present invention relates to systems and methods for compressed video data passthrough at finer levels of granularity and higher computational speed.
Video services are provided by a wide array of video content suppliers. For example, residential digital video services may include digital television, video on demand, Internet video, etc.—each service having hundreds of programs. A program refers to one or more bitstreams that are used to represent the video content and associated audio content. A target receiver for the programs, such as a set-top box (STB) located in a residential home, receives video programs from a number of different video content suppliers via one or more transmission channels.
There are presently a variety of different communication channels for transmitting or transporting video data. For example, communication channels such as coaxial cable distribution networks, digital subscriber loop (DSL) access networks, ATM networks, satellite, terrestrial, or wireless digital transmission facilities are all well known. In fact, many standards have been developed for transmitting data on the communication channels. For the purposes herein, a channel is defined broadly as a connection facility to convey properly formatted digital information from one point to another. A channel includes some or all of the following elements: 1) physical devices that generate and receive the signals (modulator/demodulator); 2) medium that carries the actual signals; 3) mathematical schemes used to encode and decode the signals; 4) proper communication protocols used to establish, maintain and manage the connection created by the channel 5) storage systems used to store the signals such as magnetic tapes and optical disks. The concept of a channel includes but is not limited to a physical channel, but also logical connections established on top of different network protocols, such as xDSL, ATM, IP, wireless, HFC, coaxial cable, Ethernet, Token Ring, etc.
The channel is used to transport a bitstream, or a continuous sequence of binary bits used to digitally represent video, audio and/or general data. A bit rate is the number of bits per second that is required to transport the bitstream. A bit error rate is the statistical ratio between the number of bits in error due to transmission and the total number of bits transmitted. A channel capacity is the maximum bit rate at which a given channel can convey digital information with a bit error rate no more than a given value.
Since transmission of video data with existing communication channels is often excessive, compression is an approach that has been used to make digital video images more transportable. Digital video compression allows digitized video data to be represented in a much more efficient manner and makes it possible to transmit the compressed video data using a channel at a fraction of the bandwidth required to transmit the uncompressed video data. International standards have been created for video compression schemes. These include MPEG-1, MPEG-2, MPEG-4, H.261, H.262, H.263, H.263+, etc. These standardized compression schemes rely on several algorithm schemes such as motion compensation, transform coding (for example, DCT transforms or wavelet/sub-band transforms), quantization of the transform coefficients, and variable length coding (VLC).
Despite the bit rate savings achieved by compression, video data is still frequently altered during transmission. Transcoding refers to altering data or information in a bitstream without changing the format of the bitstream. When desirable, transcoding reduces the bit rate of the compressed video data. Thus, after transcoding, the new bit rate for the compressed video data is smaller than the original bit rate.
One drawback of compression is increased difficulty in data manipulation and handling during transmission. As a result of the compression structure, transcoding is currently done on a picture by picture basis. When bit rate reduction is desired, some pictures are transcoded, others are passed through with no alteration or rate reduction. Since some pictures are not transcoded, the other pictures are transcoded more aggressively to compensate. Unfortunately, this forces a large and unproportional amount of rate reduction on a limited set of pictures. This unequal transcoding between pictures may lead to picture quality degradation, such as a ‘beating effect’ due to the uneven quantization between consecutive frames. Here, the viewer can undesirably perceive significant changes in video quality between frames.
In addition, picture based pass through and transcoding cannot handle portions of pictures requiring no rate-reduction, but included in a picture that is selected for transcoding. These portions of the pictures may comprise a significant fraction of a picture. Unnecessary transcoding in this manner will waste the computation cycles and may also lead to further diminished video output quality.
Based on the foregoing, improved methods and systems for transmitting video data would be desirable.